1. Harbin Institute of Technology(Weihai), Department of Mathematics, Weihai, P.R. China + China University of Petroleum (East China), College of Science, Qingdao, P.R. China
2. Harbin Institute of Technology(Weihai), Department of Mathematics, Weihai, P.R. China
3. Huaiyin Normal University, School of Mathematical Science, Huaian, P.R. China